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2.2 Measuring Earthquakes

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Vocabulary

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focus-point beneath earths surface where rock that is under stress breaks.
epicenter - the point on the surface directly above the focus.
seismic waves - vibrations that travel through earth carrying the energy released during a earthquake.
P waves - earthquake waves that compress and expands the ground like an accordion .
S waves - earthquake waves that vibrate from side to side as well as up and down.
Surface waves- they produce the most server ground movements.
Seismograph- records the ground movements caused by seismic waves as they move through the earth.
magnitude- measurement of earthquake strength based on seismic waves and movement along faults.
Mercalli Scale - developed to rate earthquakes according to their intensity.
Ritcher Scale - rating the size of seismic waves as measured by a particular type of mechanical seismograph.
Moment Magnitude scale - to rate earthquakes of all sizes near or far

Outline

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Seismic Waves

• Seismic waves are vibrations that travel through Earth carrying the energy released by an earthquake.These waves race out from the focus in all directions.There are three categories of Seismic waves.They are P waves,S waves and surface waves.The earthquake's focus sends out only two types of waves:P waves and S waves.When these two waves reach Earth's surface, surface waves develope.

• Primary Waves
  • Primary waves or P waves are the first to arrive.These earthquake waves compress and expand the ground and cause buildings to contract and expand.

• Secondary Waves
  • Secondary waves or S waves arrive right after P waves.They shake the ground back and forth and when they reach the surface they shake structures viciously.But unlike P waves which can travel through both solids and liquids,S waves cannot travel through lquids.

• Surface waves
  • When P waves and S waves reach Earth's surface, surface waves develope.Though surface waves move more slowly than P waves and S waves ,they produce the most damage.

Detecting Seismic Waves

Measuring Earthquakes

• Magnitude is a measurement of an Earthquake's strengthstrrength based on seismic wavesand movement along faults.There are three ways of measuring earthquakes.They are the Mercalli Scale, Ritcher Scale and the Moment Magnitude scale.

• The Meralli Scale
  • The Mercalli Scale was developed in the early twentieth centuary.It rated Earthquakes according to their intensity.Though it didn't give a percise measurement,the twelve steps of The Meralli Scale helped describe how the earthquake damaged people,buildings and the land surface.

• The Richter Scale
  • The Richter Scale rated earthquakes according to the size of their seismic waves.However the Richter Scale provided accurate information for small nearby earthquakes only.

• The Moment Magnitude Scale
  • The moment magnitude scale can be used to rate earthquakes of all sizes,near or far.It is a rating system that estimates the total energy released by an earthquake.

Locating the Epicenter

• In order to locate the epicenter Geologists find the differene between the arrival times of P waves and S waves.Then they draw at least three circles using data from different seismographs set up in stations all over the world.The point where the circles intersect is the location of the epicenter.

Diagram

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This diagram shows how buildings are affected by P waves and S waves.